Plasma has long been employed to process substrates (e.g., wafers or flat panels) to form electronic products (e.g., integrated circuits or flat panel displays). Semiconductor wafers are typically placed in an etch chamber with a mask layer to direct the etch of underlying materials. The etching process removes the underlying materials not covered by the mask. Although etching processes have been well studied and precise recipes are often defined for specific structures, materials and/or material stacks, variations in etch performance still occur. The reason for this is that etching processes in real-time fabrication environments are carried out in different etch chambers. These chambers, although often times tuned and matched, still are not the physically or electrically the same. In addition, wafers being processed can vary from wafer to wafer or lot to lot. Still further, variations can be introduced by the way wafers are placed into each chamber, e.g., wafer off-set variations, wafer tilts, wafer thickness, etc.
As a result, wafer etching processes often utilize at least one type of end pointing technique. Such techniques can vary from fabricator to fabricator, but most commonly used end pointing may include time based end-pointing or optical end-pointing. Time based end-pointing relies on use of pre-calibrated estimates of when a particular etch process should end, e.g., so as to remove a predefined amount of material. Optical end-pointing systems are designed to monitor spectral emissions of the plasma or reflections off the wafer, in an attempt to identify when changes in the spectral emissions are indicative of a change of etch material. For example, if the etch has removed all of a certain amount of material, or when a different material starts to be removed, the spectral emissions at the point in time will change. Unfortunately, current techniques still suffer in accuracy, which is challenged even further with the ever shrinking feature sizes. In regard to current optical end-pointing, the use of changes in optical conditions is reliant on spectral conditions of one specific point in time.
It is in this context that embodiments arise.